1. Field of the Invention
This invention pertains generally to bolted fastener joints for use at high temperatures, and more particularly to a fastener which maintains high preload levels at elevated temperatures when used to join high-temperature refractory materials.
2. Description of the Related Art
Various types of fasteners have been developed which are designed to maintain joint clamp-up at high temperatures by compensating for dissimilar thermal strains when the fastener and the structures being joined are made from materials with different coefficients of thermal expansion. These fasteners are designed to provide a snug fit between the structures which they connect (typically low-thermal expansion materials) at all temperatures during extreme temperature cycling without producing excessive thermal stress. For example, U.S. Pat. No. 4,512,699 to Jackson et al. discloses a "DAZE" fastener for use when the fastener parts and structural elements have substantially different coefficients of thermal expansion. The DAZE fastener system provides frusoconical abutting surfaces between the structural elements and fastener parts to translate any differences in thermal expansion or contraction between the parts into a sliding motion to avoid deleterious thermal stresses in the joint. The DAZE fastener is designed to allow unrestrained expansion of the fastener parts away from the coincident verticies of the cone formed by the frustoconical fastener head and a plane defined by the mating surface between the fastener's metal washer and spacer washer. With the exception of the DAZE fastener's spacer washer, which is formed of the same material as the structural elements, the entire DAZE fastener is formed of metal or alloy.
U.K. Patent No. GD 2,087,503 A discloses a fastener which includes a thermally responsive spacer, such as a washer, with at least two parts which have mutually contacting faces inclined with respect to the direction of the tensile force. The two parts are made of materials with different coefficients of thermal expansion chosen such that the two members expand or contract relative to one another to compensate for the differential thermal expansions of the components being fastened, and the fastener bolt. Both this and the DAZE fastener utilize intricate frustoconical sliding surfaces which compensate for the differences in thermal expansion. Those relatively lengthy parts in both fasteners which bear tensile force are formed of metal.
The coefficient of thermal expansion for metals ranges from 3.times.10.sup.-6 to 13.times.10.sup.-6 in./in./.degree.F., with the coefficient of thermal expansion for steel being about 10.times.10.sup.-6 in./in./.degree.F. In contrast, ceramic materials have coefficients of thermal expansion of about 1.times.10.sup.-6 to 4.times.10.sup.-6 in./in./.degree.F. Carbon-carbon (C--C) composite materials also may have low coefficients of thermal expansion. When connecting sheets or panels of low thermal expansion materials, such as ceramics or C--C which are used at high temperatures (e.g. above 1500.degree. F.), it is desirable to utilize a fastener with similar thermal expansion characteristics. Additionally, it is desirable that the fastener does not require possibly deleterious countersinking in the structural elements, or frustoconical fastener surfaces which Preclude simplicity of design. The fastener should be moderately self-aligning thereby requiring less tolerance control, and should not require that the holes through the fastened structures be drilled or reamed simultaneously. The fastener also should be capable of connecting fairly thin pieces of material with thickness below 0.25 inches, and should be significantly lighter and stronger than current designs.